Pancake Boxes and Ceiling Fixtures

Q.To retrofit light fixtures
for old plaster ceilings that will be covered with new
drywall, I plan to screw 1/2-inch-deep metal pancake
boxes into the lathe of the existing ceilings and hang
the new drywall around the boxes. But my electrician
wants to cut into the ceiling and install deeper
ceiling boxes, which will require additional blocking
and more labor. He says that 3 1/2-inch-diameter by
1/2-inch-deep pancake boxes don’t have
sufficient fill capacity to make the electrical
connection; is he right?

A.Harlan Madsen of South
Side Electric, an electrical contractor in
Bloomington, Minn., responds: A single 14/2 NM
feed with three conductors (hot, neutral, and
ground) would require a box with a 6-cubic-inch
capacity (2 cubic inches for each conductor), while
a 3 1/2-inch pancake box has only a 3.9-cubic-inch
volume allowance (2008 NEC 314.16).

A 4-inch-diameter pancake box has a 6-cubic-inch
volume allowance, enough for a single light fixture
but not enough if you’re using a 12/2 feed
or making a splice — and not enough if the
fixture has a mounting device that occupies space
in the box. The NEC does allow the canopy of the
fixture to add to the cubic inch capacity of the
box, but the volume must be indicated on the
canopy.

Because of these limitations, we install 3
1/2-inch pan boxes only if the fixture specifically
requires it, or in cases where there will be
transition to wire mold and an extension can be
added to the box; I don’t recommend them
under any other circumstances.

A final complication is that the National
Electrical Code now requires ceiling boxes to be
rated to carry 50 pounds (2008 NEC 314.27[A])
— and I doubt that screwing the box to the
lathe will meet that requirement. Your electrician
is right in calling for additional blocking, and
you might as well use bigger boxes, too.